This invention relates to a plunger lift system for moving liquids upwardly in a hydrocarbon well.
There are many different techniques for artificially lifting formation liquids from hydrocarbon wells. Reciprocating sucker rod pumps are the most commonly used in the oil field because they are the most cost effective, all things considered, over a wide variety of applications. Other types of artificial lift include electrically driven down hole pumps, hydraulic pumps, rotating rod pumps, free pistons or plunger lifts and several varieties of gas lift. These alternate types of artificial lift are more cost effective than sucker rod pumps in the niches or applications where they have become popular.
One of the developments that has evolved over the last thirty years are so-called tubingless completions in which a string of tubing, usually 2xe2x85x9exe2x80x3 O.D., is cemented in the well bore and then used as the production string. Tubingless completions are never adopted where pumping a well is initially considered likely because sucker rod pumps have proved to be only slightly less than a disaster when used in a 2xe2x85x9exe2x80x3 tubingless completions. Artificial lift in a 2xe2x85x9exe2x80x3 tubingless completion is almost universally limited to gas lift or free pistons. Thus, tubingless completions are typically used in shallow to moderately deep wells that are believed, at the time a completion decision is made, to produce all or mostly gas, i.e. no more liquid than can be produced along with the gas.
Gas wells reach their economic limit for a variety of reasons. A very common reason is the gas production declines to a point where the formation liquids are not readily moved up the production string to the surface. Two phase upward flow in a well is a complicated affair and most engineering equations thought to predict flow are only rough estimates of what is actually occurring. One reason is the changing relation of the liquid and of the gas flowing upwardly in the well. At times of more-or-less constant flow, the liquid acts as an upwardly moving film on the inside of the flow string while the gas flows in a central path on the inside of the liquid film. The gas flows much faster than the liquid film. When the volume of gas flow slows down below some critical value, or stops, the liquid runs down the inside of the flow string and accumulates in the bottom of the well.
If sufficient liquid accumulates in the bottom of the well, the well is no longer able to flow because the pressure in the reservoir is not able to start flowing against the pressure of the liquid column. The well is said to have loaded up and died. Years ago, gas wells were plugged much quicker than today because it was not economic to artificially lift small quantities of liquid from a gas well. At relatively high gas prices, it is economic to keep old gas wells on production. It has gradually been realized that gas wells have a life cycle that includes an old age segment where a variety of techniques are used to keep liquids flowing upwardly in the well and thereby prevent the well from loading up and dying.
There are many techniques for keeping old gas wells flowing and the appropriate one depends on where the well is in its life cycle. For example, the first technique is to drop soap sticks into the well. The soap sticks and some agitation cause the liquids to foam. The well is then turned to the atmosphere and a great deal of foamed liquid is discharged from the well. Later in its life cycle, when soaping the well has become much less effective, a string of 1xe2x80x3 or 1 xc2xdxe2x80x3 tubing is run inside the production string. The idea is that the upward velocity in the small tubing string is much higher which keeps the liquid moving upwardly in the well to the surface. A rule of thumb is that wells producing enough gas to have an upward velocity in excess of 10xe2x80x2/second will stay unloaded. Wells where the upward velocity is less than 5xe2x80x2/second will always load up and die.
At some stage in the life of a gas well, these techniques no longer work and the only approach left to keep the well on production is to artificially lift the liquid with a pump of some description. The logical and time tested technique is to pump the accumulated liquid up the tubing string with a sucker rod pump and allow produced gas to flow up the annulus between the tubing string and the casing string. This is normally not practical in a 2 xe2x85x9exe2x80x3 tubingless completion unless one tries to use hollow rods and pump up the rods, which normally doesn""t work very well or very long. Even then, it is not long before the rods cut a hole in the 2 xe2x85x9exe2x80x3 string and the well is lost. In addition, sucker rod pumps require a large initial capital outlay and either require electrical service or elaborate equipment to restart the engine.
Free pistons or plunger lifts are another common type of artificial pumping system to raise liquid from a well that produces a substantial quantity of gas. Conventional plunger lift systems comprise a piston that is dropped into the well by stopping upward flow in the well, as by closing the wing valve on the well head. The piston is often called a free piston because it is not attached to a sucker rod string or other mechanism to pull the piston to the surface. When the piston reaches the bottom of the well, it falls into the liquid in the bottom of the well and ultimately into contact with a bumper spring, normally seated in a collar or resting on a collar stop. The wing valve is opened and gas flowing into the well pushes the piston upwardly toward the surface, pushing liquid on top of the piston to the surface. Although plunger lifts are commonly used devices, there is more art than science to their operation.
A major disadvantage of conventional plunger lifts is the well must be shut in so the piston is able to fall to the bottom of the well. Because wells in need of artificial lifting are susceptible to being easily killed, stopping flow in the well has a number of serious effects. Most importantly, the liquid on the inside of the production string falls to the bottom of the well, or is pushed downwardly by the falling piston. This is manifestly the last thing that is desired because it is the reason that wells die. In response to the desire to keep the well flowing when a plunger lift piston is dropped into the well, attempts have been made to provide valved bypasses through the piston which open and close at appropriate times. Such devices are to date quite intricate and these attempts have so far failed to gain wide acceptance.
Disclosures of some interest relative to this invention are U.S. Pat. Nos. 2,074,912 and 3,090,316.
Co-pending application Ser. No. 09/312,737 discloses a plunger lift with a multipart piston. Although this system has worked surprisingly well, it is possible to improve the efficiency, reliability and durability of a multipart piston of a plunger lift.
In this invention, a multipart piston includes a ball and a sleeve that are independently allowed to fall inside the production string toward the productive formation. The cross-sectional area of the ball and sleeve are such that upward flow of gas is substantially unimpeded and the pieces fall through an upwardly moving stream of gas and liquid. Thus, the piston of this invention is normally dropped into a well while it is flowing. This has a great advantage because the liquid in a film on the inside of the production string does not fall into the bottom of the well.
When the ball nears the bottom of the well, it falls into any liquid near the bottom of the well and contacts a bumper assembly which cushions the impact of the device. Ideally, the plunger lift is being dropped frequently enough so there is no liquid column in the bottom of the well so the ball falls directly on the bumper assembly. In this invention, the bumper assembly includes a spring having coils that open upwardly to receive the ball, i.e. there is no anvil for the ball to contact. When the sleeve reaches the ball, they unite into a single component that has a cross-sectional area comparable to existing plunger lift pistons, i.e. any gas entering the production string from the formation is under the piston and pushes it upwardly, thereby pushing any liquid upwardly in the well to the surface.
The sleeve provides a central passage through which the gas flows as the sleeve falls in the well. The ball is sized to close the central passage and provides a second piece of the piston. The flow passage around the ball is on the outside as the it falls in the well. A ball appears to be an ideal shape for one of the components of a two part piston of a plunger lift because repeated impacts are not concentrated in any one location so wear is spread around.
When the united components reach the well head at the surface, a decoupler separates the sleeve from the ball in much the same manner as that disclosed in co-pending application Ser. No. 09/312,737. The ball accordingly immediately falls toward the bottom of the well. Conveniently, a catcher holds the sleeve and then releases the sleeve after the ball is already on the way to the bottom or after a delay period that is used to control the cycle rate of the plunger lift.
Plunger lift pistons of this invention made of conventional steels have proved quite successful in most wells. Some wells present such a difficult problem that the pistons have worn more quickly than desired. An analysis of the problem suggests that, in these difficult wells, the ball and sleeve are reaching the bottom of the well when there is no liquid column in the well, i.e. all of the liquid is in a film flowing upwardly on the inside of the production string. Because the ball and sleeve are reaching the bottom when there is no liquid in the well, they are travelling at high speeds. The force acting on either the ball or the sleeve is the mass of the element multiplied by the square of its velocity. From a production standpoint, it is desirable that no liquid column build up in the bottom of the well but this is not desirable from the standpoint of providing a long lived plunger piston.
One aspect of this invention is to provide a lighter sleeve and piston which reduces the applied force when the element reaches the bottom of the well. Because the sleeve and piston have to have substantial strength, aluminum alloys have proved unsuccessful. Sleeves and balls made of titanium alloys have proved much lighter than steel components and have proved to be much longer lived in use.
It is an object of this invention to provide an improved plunger lift and more particularly an improved two part plunger piston.
A more specific object of this invention is to provide a multipart piston for a plunger lift including a ball which is dropped first into the well and a sleeve sized to receive and unite with the ball near the bottom of the well and then move upwardly as a unit to move liquids toward the surface.
A further object of this invention is to provide a plunger lift piston made of a titanium alloy.
These and other objects of this invention will become more fully apparent as this description proceeds, reference being made to the accompanying drawings and appended claims.